The present invention relates to a high hardness rubber composition and a tire using the same. More specifically, the present invention relates to a high hardness rubber composition useful as a rubber composition for a tire and a tire using the same.
As run-flat tires employing a high hardness reinforcing rubber for a sidewall have come into practical use, these tires make it possible to drive certain miles even in the case of blowout where the air pressure falls to zero. Then it has become unnecessary for a vehicle to carry a spare tire at all times, which means a reduction of the overall weight of a vehicle is achieved. As to run-flat tire driving, however, the speed and distance are still limited and further improvement of run-flat durability is demanded.
As an effective means for improving run-flat durability, there is a process for increasing the thickness of a reinforcing rubber. However, according to this process, the tire gets heavy whereas the initial purpose of using a run-flat tire is weight reduction.
Other useful processes for improving run-flat durability include the prevention of heat build up by diminishing distortion of a reinforcing rubber itself caused by bending which occurs during run-flat driving. For example, there is a process for increasing the amount of a reinforcing filler such as carbon black. However, the filler has great load on the kneading step, and in addition, heat build up characteristic is increased. Thus, improvement of run-flat durability cannot be expected according to this method.
Another possible process for preventing heat build up of a rubber in response to distortion by external force is to increase the amount of sulfur or a vulcanization accelerator.
When a large amount of low molecular weight powdery sulfur is added, sulfur blooms on the rubber surface before vulcanization, causing lowering of adhesion in the forming step of tire. Therefore, it is generally known to use high molecular weight sulfur called insoluble sulfur. This insoluble sulfur can remain in a rubber as high molecular weight sulfur before vulcanization, while it decomposes into low molecular weight sulfur and can be vulcanized in the same manner as usual low molecular weight sulfur in a hot atmosphere of vulcanization. However, even if insoluble sulfur is used, it decomposes into low molecular weight sulfur due to heat build up and the like during the extrusion step, and blooms on the rubber surface when used in a large amount. For these reasons, it is desirable that the amount of insoluble sulfur is reduced as much as possible when it is used from the viewpoint of processability.
On the other hand, there is a process called effective vulcanization, wherein a small amount of sulfur is mixed with a large amount of a vulcanization accelerator to impart low heat build up characteristics as mentioned above. However, typically used sulfen amide accelerators such as TBBS (N-tert-butyl-2-benzothiazolylsulfenamide) and CBS (N-cyclohexyl-2-benzothiazolylsulfenamide) have low solubility in rubber. Therefore, when the sulfen amide accelerator is used in a large amount, it blooms on the rubber surface before vulcanization, causing lowering of adhesion in the forming step of tire as in the case of sulfur mentioned above.
Meanwhile, a high hardness compound is required also for bead apex of a pneumatic tire to achieve rigidity of a tire concerning controllability. Conventionally, high hardness of bead apex is achieved by adding a thermosetting resin to the composition. However, when such a thermosetting resin is used, there has been a problem that rolling resistance of a tire is increased because of increasing heat build up characteristic.
An object of the present invention is to improve run-flat durability and to prevent precipitation of sulfur and a vulcanization accelerator on the rubber surface before vulcanization by adding sulfur and vulcanization accelerators to a reinforcing rubber in appropriate amounts. Another object of the present invention is to improve rigidity of a tire and reduce rolling resistance simultaneously by adding sulfur and vulcanization accelerators to a reinforcing rubber in appropriate amounts.
That is, the present invention relates to a high hardness rubber composition comprising 2 to 8 parts by weight of sulfur and at least 5 parts by weight of two or more vulcanization accelerators based on 100 parts by weight of a rubber component, at least one of said accelerators being a sulfen amide accelerator. The present invention also relates to a run-flat tire using the rubber composition for a reinforcing rubber, and a pneumatic tire using the rubber composition for bead apex.